1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus suitable for an apparatus such as an image scanner, a copying machine, or a facsimile, in which, even if originals are those having a wide variety of original sizes, e.g., films (objects), the space for an illuminating means can be small enough to allow extremely efficient illumination, and the image information of these originals also can be read with a high accuracy.
2. Related Background Art
In a conventional image reading apparatus such as an image scanner, a copying machine, or a facsimile, an image of a light beam reflected by a reflection original or an image of a light beam transmitted through a transmission original film, for example, is formed by an imaging means on the surface of a photoelectric converting means, such as a line sensor (CCD), in which a plurality of pixels (light-receiving elements) are arranged one-dimensionally, thereby reading the image information of that original.
In the above apparatus, detection data from the light-receiving elements corresponding to pixels are sequentially, electrically scanned (converted) in the longitudinal direction (main scan direction) of the line sensor. Meanwhile, in the lateral direction (subscan direction) of the line sensor, an original image (original film) and the line sensor are mechanically moved to scan relative to each other. Consequently, the image information of that original is two-dimensionally read.
If a wide variety of original sizes are to be handled, a plurality of imaging lenses of each different imaging magnification are provided to obtain matching between these original sizes and the detection line length of a line sensor. That is, the imaging lenses are switched in accordance with the size of a given original to permit a photoelectric converting means, such as a line sensor (CCD), to read the image information of that original.
Alternatively, a plurality of original illuminating means (light sources) having different illumination regions corresponding to original sizes may be provided. In this case, an illumination region of a given original is illuminated with a desired illumination quantity by switching the original illuminating means in accordance with the size of that original, and a photoelectric converting means, such as a line sensor (CCD), reads the image information of the original.
FIG. 1 is a schematic view showing main parts of a conventional image reading apparatus using a transmission original as an object to be projected.
Referring to FIG. 1, an original table (original support table) 501 is arranged horizontally at a fixed position of the apparatus main body. This original table 501 has a light-transmitting plate structure, and various original films F with different sizes can be placed on it. For example, the original films F from a 35-mm film (36 mm.times.24 mm) F1 to a 4.times.5-inch film (4 inches.times.5 inches) F2 can be placed.
An illuminating means 502 is arranged above the original table 501 in FIG. 1. The illuminating means 502 is so designed as to be able to evenly illuminate an entire region to be read of the original film F of the maximum size, e.g., the 4.times.5-inch film F2. In this case, it is necessary to be able to illuminate at least a circular region whose diameter is the length of the diagonal of the 4.times.5-inch film F2. Therefore, an illuminating system is constituted by using a large light source 503 and by increasing an illuminating optical path length L0.
An imaging means 510 has several different exchangeable imaging lenses of each different imaging magnification. Of these imaging lenses, a first imaging lens 505 is arranged below the original table 501 when a relatively small original (small-sized original), e.g., the 35-mm film F1 is placed on the original table 501. The first imaging lens 505 images the image information of the 35-mm film F1 on the surface of a line sensor (CCD) 508, as a photoelectric converting means, arranged below the first imaging lens 505.
The imaging magnification of this imaging lens 505 is set such that the width of the 35-mm film F1 matches the detection line length of the line sensor 508.
A second imaging lens 506 is arranged below the original table 501 when a relatively large original (large-sized original), e.g., the 4.times.5-inch film F2 is placed on the original table 501. The second imaging lens 506 images the image information of the 4.times.5-inch film F2 on the surface of the line sensor 508.
The imaging magnification of this second imaging lens 506 is so set that the width of the 4.times.5-inch film F2 matches the detection line length of the line sensor 508.
A third imaging lens 507 is used for an original film F3 with a size intermediate between the sizes of the original films F1 and F2. The imaging magnification of this third imaging lens 507 is set on the basis of the same design concept as for the first or second imaging lens 505 or 506.
In the conventional image reading apparatus as mentioned above, the first, second, and third imaging lenses 505, 506, and 507 are so supported as to be switched below the original table 501 in FIG. 1. For example, the imaging lenses 505, 506, and 507 are switched in accordance with the size of a given original film F to image the image information of that original film F on the surface of the line sensor 508.
Simultaneously, the line sensor 508 is moved to scan by a linear moving mechanism (not shown) in the direction (subscan direction) indicated by an arrow A0 in FIG. 1 in a plane parallel to the original table 501, thereby two-dimensionally reading the image information of the original film F of that size.
Note that, for example, the switching between the imaging lenses is manually performed by an operator while checking the size of the original film F to be used.
In the conventional image reading apparatus illustrated in FIG. 1, the light source 503 used as an illuminating means has an illumination region set on the basis of the large-sized original F2. In addition, in the imaging means 510, one of a plurality of the imaging lenses is selectively used in accordance with the original size. Consequently, the following problems are encountered.
1. A large light source is required to two-dimensionally, evenly illuminate the image information of an original of a large size (large-sized original) throughout the entire region to be read. In addition, the size of a power source for supplying power to this light source is also increased to increase consumption power. Also, since the field angle of an illumination region must have a size corresponding to the length of the diagonal of an original, the illuminating optical path length increases accordingly. Consequently, the illuminating means becomes large and expensive.
2. In reading the image information of originals by switching a plurality of imaging lenses in accordance with the sizes of the originals, it is necessary to set the distances between the imaging lens, the original, and the photoelectric converting means, and the right angle of the optical axis to the original surface with an extremely high accuracy, in order to maintain the quality of the read images with a high accuracy. To this end, the imaging system must be supported by a highly rigid structure, and a positioning mechanism processed extremely precisely is required. The result is a very large, expensive structure.